This article has been updated. It is accurate as of 9/23/20.
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For commercial operations that rely on effective heat transfer, external corrosion of heat exchanger components is a major concern. When left unchecked, corrosion can significantly reduce a coil's effectiveness, but sometimes it’s difficult to know which anti-corrosion option best meets your needs.
Fortunately, manufacturers offer a variety of corrosion-resistant products, including pre-coated finstock, full-coil coatings, spray-on topcoats, and baked-on epoxies. This post will focus on advantages and tradeoffs of all four options, while outlining some suitable applications for each.
Option #1: Electrofin E-coat
Also known as electrocoating or cathodic deposition, such as Electrofin (a PPG product), e-coating is one of the most effective means of corrosion protection available. The ElectroFin coating process involves electrically charging an assembled coil, then submerging it into an oppositely charged bath of water and paint solids. The coil essentially becomes a magnet to which the paint is attracted (think powder-coating, but underwater).
The coil is then rinsed and oven-baked, resulting in a very even hydrophobic coat with uniform dry film thickness of anywhere from 0.6 to 1.2 mil. This includes fin edges, meaning the coil is fully encapsulated and protected. When properly maintained, E-coat can provide effective corrosion protection for years.
Advantages: E-coat is an effective means of protecting a coil’s joints from galvanic corrosion, which occurs when dissimilar metals interact over time in the presence of an electrolyte. The difference in the electrode potential of the two metals results in one acting as a cathode, and the other serving as an anode, which makes for an accelerated corrosion of the anode metal. This corrosion weakens the connection at the coil’s joints, reducing its effectiveness. E-coat provides insulation between the two metals, effectively creating a barrier that hinders the occurrence of the galvanic action.
E-coating also leaves the coil resistant to corrosion caused by other sources, even relatively strong acids and bases.
Tradeoffs: E-coating a coil is more expensive than using pre-coated finstock, and its hydrophobic nature makes it unsuitable for applications where the presence of beaded water could have negative effects (like in an evaporator). E-coating also requires the coil to be shipped to a coater, coated, then shipped back, which adds lead time.
Applications: E-coat provides robust defense against corrosion, withstanding 6,000+ hours during ASTM B117 tests. This resilience makes it well-suited for HVAC use in coastal areas. It’s also frequently used in heat exchangers on ships and other marine environments for the same reasons. See the chart below for e-coat’s performance on other tests.
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Option #2: Alucoat Waterfin 700
Waterfin 700 is a hydrophilic pre-coated finstock product manufactured for use in the production of heat exchangers. It’s coated in a silica-free lacquer and is available in several thicknesses, alloys, and tempers.
Advantages: Like E-coat, Waterfin 700 protects against galvanic corrosion, too. However, it does so by eliminating contact between dissimilar metals. By using collars that flare out, the coil’s tubes never come in contact with any uncoated fin material, preventing a dielectric circuit and impeding the galvanic reaction. Waterfin 700’s film also covers the area of the fin away from the joint, providing moderate corrosion protection across the remainder of the fin.
Price point is another primary benefit of pre-coat, as it’s cheaper than e-coat. Also, unlike electrocoating, Waterfin 700 is ready to use right away. It’s sold in reels like non-coated finstock, so there’s no need to send anything to a coater, which saves lead time.
Tradeoffs: Waterfin 700 provides a lower level of corrosion defense than E-coat, withstanding 1000 hours in ASTM B117 tests, and is appropriate for heat exchangers in less extreme corrosive environments.
Applications: The hydrophilic nature of Alucoat’s Waterfin 700 makes it suitable for environments like evaporators, where water needs to flow, rather than bead. Waterfin 700 would be a good choice for use in heat exchangers in food processing or residential and commercial HVAC applications. Waterfin 700 also provides corrosion protection from chemicals and pollutants, and would hold up well in environments where either are present.
Option #3: E-coat + spray-on hydrophil topcoat
A third option involves applying a hydrophilic spray-on coating product after the coil has been E-coated. Upon curing, the hydrophilic coating develops an ultrathin hydrophilic surface where water is attracted to the surface of the substrate, resulting in condensate draining off the bottom of the coil rather than blowing off the coil face.
Advantages: Not only does the coil benefit from the corrosion resistance and overall strength of the E-coat, the addition of the hydrophilic layer allows the coil to be used in applications that require water to flow rather than bead. This coating provides marked improvement in evaporator condensate removal without carryover, while also increasing air velocity up to 26% compared to an uncoated fin. The hydrophil topcoat also provides its own moderate abrasion resistance without loss of hydrophilicity.
Tradeoffs: Aside from a minor added cost (1.5 – 2%), the tradeoffs of adding a hydrophilic topcoat are minimal. The topcoat is applied at the same facility where the coil is E-coated, so there’s no additional lead time or added freight cost.
Option #4: Heresite P-413 Baked Phenolic Epoxy
A fourth corrosion-resistant option is a baked-on phenolic coating manufactured by Heresite and designed specifically for use on heat exchanger external surfaces. It can be applied via full immersion, flood, or spray methods.
Advantages: Due to its dense cross linking, Heresite P-413 can provide high levels of corrosion resistance even when applied as thin film. Heresite also holds an ANSI 51 certification for non-contact food zones, while also providing resistance to a multitude of chemicals.
Tradeoffs: The added step can contribute to increased lead time (around 10 days compared to e-coat’s 7, but Heresite does offer expedited options). As far as thickness goes, Heresite and E-coat are fairly similar, with each having a dry film thickness of roughly 1 mil.
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Applications: Heresite P-413 is designed for use on components and parts in air conditioning and refrigeration system, especially those that operate in moderate to severely corrosive environments, such as marine and offshore applications, among others.
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The coil coating process (see Diagram 1) provides a way to rapidly apply a variety of paints in a uniform, controlled, continuous and quality fashion at line speeds varying from 100 to 700 feet per minute with the paint being cured in 15-45 seconds to achieve a Peak Metal Temperature of 390 to 480°F depending on the paint and metal thickness. In the coil coating process, the metal coil is first unwound, cleaned and pre-treated, one or more layers of paint are applied on a flat continuous sheet, heat cured, cooled and rewound for shipment. At the fabricator, it is then cut to the desired size and formed into its finished shape.
Since the metal is first painted and then formed into a part, coil coatings are frequently referred to as pre-paint. Pre-paint refers to the process of painting metal before fabrication rather than painting a formed article. The coated product must not only be able to be cured quickly, it must also be flexible and resistant to pressure mottling as coated coils can weigh several tons.
Primary coil coated metal substrates include:
- Steel
- Cold rolled and hot rolled steel
- Galvanized steel (zinc coated steel — hot dip and electrogalvanized)
- Galfan (~95% zinc and 5% aluminum coated steel)
- Galvalume (43.5% zinc, 55% aluminum-with 1.5% silicon coated steel)
- Aluminum
Compared to most other application methods, coil coating is nearly 100% efficient from a paint usage standpoint. As opposed to a spray-applied coating, a coil-coated product offers uniform film thickness rather than the thicker films on edges, corners and bends that are more typical of spray-applied coatings.
Benefits of coil coating:
- Consistent quality
- High speed
- Nearly 100% efficient, therefore lower cost
- High quality
- Lower environmental impact
- Wide range of metal types
- A wide variety of coating types available
- Flexibility
Topcoats are applied by reverse roll coat in which the applicator roll travels in the reverse direction of the strip and thus provides a smoother film with fewer defects. Primers and backers are normally applied by direct roll coating. Some lines also apply coil coatings using an extruder or via a solid block of paint with a softening point such that it can be applied smoothly when heated.
Coil coating physical types include solvent borne, waterborne, powder and solid block paints.
Schematic 1 – Paint layers in a typical exterior surface
Table I – Synopsis of Coil Coating Options
The total global market size of coil coatings is in the range of $3.5 billion (USD) with polyester coatings making up greater than 60% of the total. The types of paint curing employed in the coil industry include thermal, infrared, induction and UV cure. By and large, the vast majority of coil coatings are cured using gas-fired ovens. Accordingly, the remaining portions of this article will focus on thermal-cured coil coatings.
Coil primers and backers are normally applied much thinner than spray-applied liquid or powder coatings, dip or electrocoat paints. Applied primer dry film thicknesses are normally in the range of 4-6 microns, whereas topcoats are normally applied to provide a dry film thickness of 18-20 microns.
Special consideration when formulating coil coatings include adjustments in the resin technology, crosslinking, solvent system, catalyst, surface modifiers and lubricants to accommodate proper cure, flow and leveling, and minimization of surface defects. Once cured, coil coatings can offer excellent handling, physical, chemical and environmental properties. Table II details typical materials utilized in a polyester coil coating.
Table II – Raw Materials (on Resin Solids) used in a Typical Polyester Coil Topcoat (1.2:1.0 Pigment:Binder, Dupont R960 Titanium Dioxide)
In summary, coil coating technology has grown more rapidly than more conventional application technologies and is projected to grow at a rate in excess of 5% per annum due to a variety of benefits that include cost savings, versatility, environmental, performance and quality issues.
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